The 2018 midterms will be fiercely fought, with a focus on control of the U.S. House after years of Republican dominance. At the same time, the U.S. Supreme Court will likely hear a challenge to partisan gerrymandering in Gill v. Whitford later this year. And both parties gearing up for the new national redistricting cycle starting in 2021. Congressional maps are back in the political spotlight.

Today, a new report from the Brennan Center for Justice at NYU School of Law finds that extreme partisan bias in congressional maps account for at least 16-17 Republican seats in the current Congress –a significant portion of the 24 seats Democrats would need to gain control of the House in 2020 – and that only a small number of swing states account for the vast majority of this partisan skew.

Extreme Maps, the first in-depth report to use data from the 2016 election as well as the 2014 and 2012 cycles, focuses on the most egregious map-drawing abuses. The congressional maps in question have high levels of “partisan bias” – the degree of systematic advantage one party receives over another in turning votes into seats – under at least three widely accepted statistical measures. Among the findings:

This decade’s congressional maps consistently benefit Republicans: In the 26 states that account for 85 percent of congressional districts, Republicans derive a net benefit of at least 16-17 congressional seats in the current Congress from partisan bias – significantly more than previously thought.

Extreme maps have proved remarkably durable: Typically, the impact of biased maps drawn by state legislators lessens over the years as voters move, populations change, and legislators retire. But this time, the maps’ high levels of partisan bias have persisted through both pro-Republican and pro-Democratic elections.

Just six swing states and Texas account for almost all the bias: Michigan, North Carolina, and Pennsylvania consistently have the most extreme measures of bias, accounting for at least 7-10 Republican seats in each election since the 2011 redistricting. Florida, Ohio, Texas, and Virginia jointly account for most of the remaining extra Republican seats. All these states had one-party control of the process.

Maps drawn by independent commissions, courts, or split-party state governments had less bias: There is strong evidence that less partisan redistricting processes have led to less biased maps; no maps drawn since 2011 by these means had high level of bias across all three election cycles since. Conversely, maps in each of the seven states with the worst gerrymandering abuses were drawn under single-party control.

“Gerrymandering can mean many things to many people,” said Michael Li, senior counsel in the Brennan Center’s Democracy Program and co-author of Extreme Maps. “Among the most dangerous forms of gerrymandering is when redistricting locks in an unfair share of seats for one party. Courts have struggled in the past to stop this sort of abuse because it’s been hard to measure. But the last three elections show clear, measurable evidence that pernicious map-drawing abuses are a feature in a few key states – giving the courts the impetus and information they need to act. ”

“Gerrymandering has been a problem since our nation’s founding,” said Wendy Weiser, director of the Brennan Center’s Democracy Program. “But today, maps in a few states are so egregiously biased in favor of one party that they can be rightfully called ‘extreme.’ These extreme maps in only a handful of swing states completely warp the composition of Congress. They are the product of a flawed, undemocratic process, which usurps the basic power of voters to choose their representatives.”